Editor's note: Chris Berdik's Mind Over Mind examines the myriad effects of expectations—whether the eager eyes of sports fans on a key player, anticipating a sip of expensive wine or the mysterious medical efficacy of a placebo. In this excerpt, Berdik describes a peculiar contagion that some scientists attribute to the placebo's harmful counterpart, the nocebo effect—in which our expectations cause harm.

The largest wave of compulsive dancing hit Strasbourg in the summer of 1518. By the end of August, hundreds of people were dancing wildly throughout the city. Town officials overruled local physicians who said the dancers should be bled, but their chosen prescription was just as alarming: more dancing! They gathered the stricken into guildhalls and even built a stage for them in the public square. They hired dancers to keep up the energy and musicians to play a lively accompaniment. Not surprisingly, the dancers kept going and kept dying. Eventually the town leaders changed their minds and deemed the dancing a curse from an angry Saint Vitus, an early Christian whom the Romans tossed into a cauldron of boiling oil and then to the lions for refusing to renounce his faith. By the fourteenth century, the Vatican declared Saint Vitus a "holy helper" who could answer the prayers of people who had epilepsy or trouble conceiving. On his feast day, it was customary to dance at his shrine. However, saints who could heal when venerated could afflict when angered, so the town's next remedy for the dancing was civic contrition—which meant cracking down on gambling and prostitution, and the banishment of those known to traffic in vice.

Mass psychogenic illnesses [MPI] are fueled by stress and flow from expectations. According to John Waller, author of A Time to Dance, a Time to Die (2008), there were at least ten dancing outbreaks in towns along the Rhine and Mosel rivers, and most of them followed periods of tremendous hardship, such as the waves of crop-killing weather and famine that preceded the 1518 epidemic in Strasbourg.

A hungry and fearful populace was primed for a freak-out, but the loss of control that followed was scripted by cultural expectations. The region's pious citizens knew well the story of Saint Vitus, and some of the first of these dancing plagues began on or near June 15, Saint Vitus' Day.

Today's versions of MPI follow more modern expectations —including fears of environmental toxins and terrorist attacks. In November 1998, for example, a teacher in a Tennessee high school came down with a headache, nausea, and shortness of breath after complaining of a "gasoline-like" smell. Soon her students began to feel sick, too. Eventually more than a hundred staff and students were taken to the emergency room. The school was evacuated and closed for two days while extensive tests were done to locate the source of toxicity. Nothing was ever found. Sometime later, questionnaires revealed that people who reported symptoms were more likely to have known or seen somebody else get sick.

Certainly, MPI should be a designation of last resort. In addition to the risk of neglecting a dangerous toxin or infection, there's the unfortunate implication that those with symptoms are either lemmings or liars. In 2011, more than a dozen teenagers, mostly girls, in Le Roy, a town outside Rochester, New York, were hit with a mysterious outbreak of uncontrollable facial tics and muscle spasms. Investigations by public health authorities found no evidence of environmental or infectious causes, leading many to suspect MPI. That suspicion didn't sit well with many of the girls and their families. They took it as a suggestion that they were faking, that there was no pill they could take to get better, and there was no one to blame but themselves. It's the same stigma that's followed placebos for centuries, of course, only applied to the nocebo in this case. If the mind causes it, then it can't be real.

As we've seen, however, the brain has many ways to make good on our expectations, both good and bad. In response to a clinician's promise, the brain releases painkillers as strong as morphine. Anxiety short-circuits anticipation and the athlete's worst fears come true. The embodied expectations of looking powerful can send our hormones racing. As Ader's saccharin-slurping rats demonstrated, the immune system can be ratcheted up and down without a word being said.

We saw in Chapters 6 and 8 how readily we take cues for our own behavior from watching others. Why couldn't placebo and nocebo effects spread socially, too? In 2007, for a rare lab study of nocebo contagion, psychologists at the University of Hull (UK) somehow recruited 120 people to test "individual reactions to environmental substances." The researchers gave everybody an inhaler that supposedly contained a toxin known to cause headaches, nausea, itchy skin, and drowsiness. In reality, the inhalers contained nothing but plain old air.

Each subject was paired with another subject who was actually an actor in cahoots with the researchers. Sometimes the actor would show symptoms, either by answering the researchers' questions or by wincing, yawning, or throat clearing, and so on. In other cases, they'd stay symptom free.

Isolated in a room, the pairs of participants were told to inhale the toxin and then to hold their breath for three seconds. Over the next hour, a researcher checked in every ten minutes to see how they were feeling. They always spoke to the confederate first, and asked about the four symptoms mentioned up front and four other unexpected symptoms— watery eyes, scratchy throat, tightening chest, and difficulty breathing. Subjects whose partners displayed phony symptoms felt sicker and reported significantly more symptoms than subjects whose partners seemed unaffected by the "toxin."

Placebos can also be contagious, as Benedetti and his colleague Luana Colloca demonstrated in 2009. Subjects enduring painful electric shocks to their hand experienced placebo pain relief when a green light on the machine delivering the shocks was illuminated and felt more pain when the light was red. They weren't told what these lights meant —in reality, they meant nothing— but they'd watched another subject, actually a confederate of the researchers, faking his way through the red and green light shocks. Notably, the placebo pain relief was much stronger for these folks than it was for another group of subjects who were told up front what the lights meant but were given no demonstration.